Within the framework of the invention documents of value are understood as card- or sheet-shaped objects, which represent for example a monetary value or an authorization and which should therefore not be producible at will by unauthorized persons. Therefore they have properties which are not easy to produce, in particular not easy to copy, whose presence indicates authenticity, i.e. production by a correspondingly authorized authority. Some important examples for such documents of value are chip cards, coupons, vouchers, checks, shares and in particular bank notes.
For reasons of design, for distinctiveness and for protection against simple falsifications documents of value are typically provided with a color design, for example more or less complex color patterns and/or colored representations and/or characters and/or combinations of characters.
When documents of value are used, they can be soiled. Within the framework of the present invention soiling particularly means a change in the color design of the document of value effected through applying or inserting substances on or in documents of value or through irradiating the documents of value with electromagnetic radiation. The soiling of documents of value can in particular occur in the form of spots, for example caused by intentionally or unintentionally applying colored or color-altering liquids, or in the form of colored markings.
Furthermore also color wear can occur. In the following color wear is understood in particular as color changes through fading, abrasion of printing ink and/or washing out of ink by means of water or other solvents for inks of the document of value. Such color wear can occur in particular in bank notes with polymer substrates, if printing inks with which the bank notes are printed do not adhere in a sufficiently stable fashion to the polymer substrates.
In order to guarantee that documents of value are distinguishable and/or that documents of value are recognizable for users in regard of their authenticity, in particular also without the aid of technical devices, it is necessary that soiled documents of value can be detected. Due to the very great number of documents of value in circulation for example in the case of bank notes, detection by machine or automatic detection is desirable.
However, automatically detecting soiling and/or color wear at a preferably high speed is rendered difficult by the fact that on the documents of value color transitions occur, which either correspond to the normal color design of the document of value or are caused by soiling and/or color wear, which do not extend over the complete surface of the document of value. Therein the color transitions do not need to be sharp, such as for example in the area of edges of an image on a document of value, but can also occur gradually over a predetermined segment on the document of value. Soiling and/or color wear consequently have to be detected.